avcodec/hevcdec: make ff_hevc_frame_nb_refs take a const pointer

libavcodec/hevc_refs.c

@@ -508,12 +508,12 @@ fail:
     return ret;
 }
 
-int ff_hevc_frame_nb_refs(HEVCContext *s)
+int ff_hevc_frame_nb_refs(const HEVCContext *s)
 {
     int ret = 0;
     int i;
     const ShortTermRPS *rps = s->sh.short_term_rps;
-    LongTermRPS *long_rps   = &s->sh.long_term_rps;
+    const LongTermRPS *long_rps = &s->sh.long_term_rps;
 
     if (rps) {
         for (i = 0; i < rps->num_negative_pics; i++)

libavcodec/hevcdec.h

@@ -544,7 +544,7 @@ int ff_hevc_res_scale_sign_flag(HEVCContext *s, int idx);
 /**
  * Get the number of candidate references for the current frame.
  */
-int ff_hevc_frame_nb_refs(HEVCContext *s);
+int ff_hevc_frame_nb_refs(const HEVCContext *s);
 
 int ff_hevc_set_new_ref(HEVCContext *s, AVFrame **frame, int poc);